#include<bits/stdc++.h>
using namespace std;
int main() {
	double a, b, c, p;
	scanf("%lf %lf %lf", &a, &b, &c);
	p = (a + b + c) / 2;
	double s = sqrt(p*(p - a)*(p - b)*(p - c));
	printf("%.1lf",s);
	return 0;
}
